1. Field of the Invention
The present invention relates to a method for manufacturing a flexible transfer body employing curable resin as a transfer material. Specifically, the present invention relates to methods for manufacturing a flexible transfer body and a flexible optical disc in which the unevenness of the thickness of a substrate and a transfer layer thereof is reduced and to a flexible optical disc manufactured by such a method.
2. Description of the Related Art
In recent years, optical discs have been widely used as recording media for recording and reproduction since they are removable and have massive storage capacity. The optical discs include a transfer layer formed on a polycarbonate substrate usually having a thickness of 1.2 mm or 0.6 mm. Information is recorded on the transfer layer and the information is reproduced. In order to condense light on the recording surface of the transfer layer used for performing recording and reproduction, it is necessary to have position accuracy on the recording surface for optical pickup upon recording and reproduction. Thus, the substrate is made flat to obtain rigidity and the optical pickup is servo-controlled to achieve the position accuracy.
Moreover, research and development of Blu-ray discs, for example, has been promoted in which the diameter of an optical spot is further reduced by increasing the numerical aperture (NA) of an objective lens and by producing laser light with a shorter wavelength so as to improve the recording capacity of the optical discs. On the other hand, in order to increase the numerical aperture (NA) of the objective lens, the surface deflection (tilt) of the substrate must be reduced. Thus, for example, the plane accuracy of a manufactured substrate is improved and a tilt (surface deflection) servomechanism is mounted for the pickup, or a thin cover layer with a thickness of about 0.1 mm is disposed on the transfer layer and a tilt (surface deflection) margin is enlarged by performing recording and reproduction on the cover layer side.
It is possible to reduce the substrate tilt (surface deflection) of an optical disc using a designed material or manufacturing method. However, the manufacturing cost of the optical discs is increased. Further, mounting the tilt (surface deflection) servomechanism for the optical pickup also increases the cost of the optical pickup.
Upon performing reproduction from the transfer layer side without passing through the substrate of the optical disc, only a length of about 0.1 mm is provided for the distance between the surface of the transfer layer and the objective lens. Thus, when a normal optical disc is rotated as a rigid body, it is necessary to reduce the surface deflection and improve the chucking accuracy of a chucking apparatus for the optical disc in order to avoid a collision between the optical disc and the objective lens. However, these are also problematic in that the costs of the optical disc and the recording/reproduction apparatus are increased.
In view of this, research and development of an optical disc has been promoted in which an optical disc 1 is made to have flexibility as shown in FIG. 4 without mechanically improving the plane accuracy of the optical disc as a rigid body. In addition, a guide 2 is disposed on the opposite side of a recording/reproduction surface including a recording layer 1a and a protection layer 1b and an objective lens 3 for optical pickup is disposed on the recording/reproduction surface side such that the optical disc 1 having flexibility is held between the objective lens 3 and the guide 2. The optical disc 1 is aerodynamically levitated (Bernoulli's method: non-contact levitation) by the rotation thereof using the guide 2, thereby stabilizing the position of the recording/reproduction surface with respect to the objective lens 3 and reducing the amount of the surface deflection of the optical disc 1 to almost zero.
Examples of conventional methods for manufacturing the substrate of the optical disc having flexibility (hereafter referred to as a flexible optical disc) include a heat press method where a surface of a flexible sheet such as a PET (polyethylene terephthalate) film is coated with thermoplastic resin or thermosetting resin, a minute pattern of concavity and convexity on a stamper is transferred and the resin is cured by heating, and a transfer layer is subsequently formed. The examples of conventional methods also include a 2P (Photo Polymerization) method (refer to Patent Document 1) where the surface of the flexible sheet is coated with ultraviolet-curable resin, the minute pattern of concavity and convexity on the stamper is transferred and the resin is cured by ultraviolet curing, and the transfer layer is subsequently formed in the same manner. The examples of conventional methods further include a method (refer to Patent Document 2) where a flexible organic sheet is heated beyond the softening point thereof, the pattern is transferred by pressure bonding using the stamper, and the sheet and the stamper are subsequently cooled and separated.
Prior art includes a direct emboss method as disclosed in Patent Document 3, for example, where a transparent film is bonded by thermocompression. In the technique, as pressure bonding is performed after the heating, it is difficult to extremely uniform thermal unevenness and stress unevenness on a surface. Thus, unevenness is likely to be generated on optical characteristics, mechanical strength, warping, and the like. In addition, it is necessary to strictly control the accuracy of parallelism among the stamper, the film, and a plate for pressure.
Here, with reference to FIG. 5, it is assumed that a stamper 6 is tilted θ with respect to a film 5 regarding the accuracy of parallelism among the stamper 6 and upper and lower plates 7a and 7b for pressure. When the size of a CD or a DVD is assumed have φ 120 mm, generated heights Hs are values as shown in numerical formula 1.When θ=one degree, H=120×sin(one degree)=2 mm.When θ=ten minutes( 10/60deg.), H=120×sin(ten minutes)=30 μm.When θ=one minute ( 1/60deg.), H=120×sin(one minute)=3 μm.  Numerical formula 1
A preformat pattern of a flexible optical disc has a depth of about several tens of nm to 0.1 μm. Thus, even when the tilt θ of the film 5 with respect to the stamper 6 is one minute, a gap of 3 μm is generated, so that even if pressure power is applied thereto, it is not considered that the gap is completely eliminated. In other words, transfer unevenness is increased.
On the other hand, the 2P method is superior in transferability and its potential in this respect is superior to other transfer methods. In the 2P method, the thickness of the flexible optical disc is a summation of the film for transfer, the transfer layer, and the like. The film per se is industrially mass-produced, so that the thickness distribution thereof is about ±1 μm. However, although a general film has a thickness amplitude of about ±1 μm, thickness in the circumferential direction greatly changes such that it has a spike-like form. Also, in a case where resin is spread on the stamper by the 2P method, and thereafter, the stamper and the film are attracted using the upper and lower plates and adhered together, if a tilt is generated between the stamper and the film, transfer unevenness is increased in the same manner as in the aforementioned direct emboss method. In particular, in the case of the 2P method, since not much pressure can be applied as compared with the direct emboss method, it is almost impossible to modify the tilt.    Patent Document 1: Japanese Patent No. 2942430    Patent Document 2: Japanese Laid-Open Patent Application No. 6-60423    Patent Document 3: Japanese Laid-Open Patent Application No. 11-273147